One of the enduring and exciting episodes of the planetary exploration came to an abrupt end Sunday when the Galileo spacecraft plunged into the atmosphere of giant Jupiter.

With its fuel supply nearly gone, the craft was intentionally commanded to the collision course with Jupiter to eliminate any chance of a future impact with Europa that could contaminate the icy moon, which is likely to have a subsurface ocean and possibly life.

An artist's concept of Galileo burning up in Jupiter's atmosphere. Credit: David A. Hardy

The battered ship Galileo was crushed, melted, and then assimilated into the planet's all-embracing atmosphere at the completion of its 35th and final circuit around the hostile Jovian system. Galileo's life ended as it impacted the gas giant in darkness just south of the equator at 1857 GMT (2:57 p.m. EDT) spacecraft-time at a speed of approximately 48.26 km per second (nearly 108,000 miles per hour).

From launch to its demise, the spacecraft traveled 4,631,778,000 km (2,878,053,500 miles) during its 14-year mission.

However, the first man-made satellite of Jupiter leaves behind a rich legacy. Apart from the few hours of data transmitted back to Earth before its dramatic high-speed descent into the planet, Galileo bequeaths a treasure trove of scientific data that has revolutionised our ideas about the king of the planets, its retinue of satellites, its dark, dusty rings and radiation-riddled environment.

"We learned mind-boggling things. This mission was worth its weight in gold," said Dr. Claudia Alexander, the seventh and final Galileo project manager.

Triumph over adversity
Eventually launched on October 18, 1989, the failure of the umbrella-shaped high-gain antenna to open, restricted Galileo's promised flood of images to a trickle. Then further disaster threatened when the onboard tape recorder stuck only eight weeks before its scheduled arrival. Fortunately, although the opportunity to image Io from close range was lost as a result of the malfunction, the mechanical blip proved to be temporary. Read more about Galileo's history here.

Galileo finally braked into orbit around the planet on December 7, 1995 and, despite the continual threat from technical glitches, it has since completed one of the most productive missions in the history of Solar System exploration.

A menagerie of moons
The two-year prime mission began with observations of the two outer Galilean moons, the gas giant and its hostile environment. Inserted into a highly elliptical 198-day orbit around Jupiter, Galileo eventually made its first satellite flyby on June 27, 1996 sweeping past Ganymede at a distance of 835 km (519 miles) - 70 times nearer than either of its Voyager predecessors.

To everyone's surprise, Galileo's instruments revealed that the largest moon in the Solar System possessed a magnetic field. Close-up images showed prominent ice mountains and ridges that looked as if a giant rake had been dragged across the surface, while large fractures snaked across the ancient plains.

Next to be explored was cratered Callisto, a world that seemed to have changed little over billions of years. However, Galileo showed that even this inactive moon undergoes subtle changes. A blanket of dark material seems to have smoothed out the wrinkles and smaller impact features, while the rims of larger craters have slumped to expose a bright, icy basement.

Subsequent attention switched to the icy moon Europa as the CCD camera provided the most detailed images ever obtained of the strange, curved bands and 'icebergs' that etched its surface.

Galileo's view of Europa. Credit: NASA/JPL

"In some places, the ice is broken up into large pieces that have shifted away from one another, but obviously fit together like a jigsaw puzzle," said Galileo imaging scientist, Ron Greeley. "This shows the ice crust has been, or still is, lubricated from below by warm ice or maybe even liquid water."

After Galileo's prime mission came to an end in December 1997, the spacecraft was granted a two-year extension - known as the Galileo Europa Mission, because its main purpose was to delve deeper into the secrets of the mysterious ice world.

Further flybys convinced most scientists that the smallest of the Galilean moons boasts a subsurface, saltwater ocean that might support alien forms of life. Just as surprising was the discovery that similar oceans may be hidden deep beneath the icy crusts of both Ganymede and Callisto.

World of fire
Galileo's first colour image of Io was taken on June 25, 1996 at a distance of 2.2 million km (1.4 million miles). Subsequent long-range views confirmed that major changes had taken place on the volcanic moon since the Voyager flybys 17 years earlier. Clearly visible were the volcanic plumes blasted outwards from violent eruptions and ever-varying patterns of colourful, sulphurous deposits around major centres of activity.

An artist's concept of Galileo over Io. Credit: David A. Hardy

However, it was not until October 1999 that scientists were prepared to risk the spacecraft by exposing it to the intense radiation in the vicinity of Io. High-resolution images (5 to 500 m per pixel) provided spectacular views of lava in many forms - lakes, flows, fountains and curtains. The temperature of the molten rock reached a sizzling 1,700 deg C, making it the hottest lava observed anywhere in the Solar System.

Galileo also imaged huge mountains (some more than twice the height of Everest) plateaus and collapsed calderas, further evidence of Io's tectonically active crust.

Giant Jupiter
Although most attention was focused on the menagerie of moons, turbulent Jupiter also came under scrutiny. Images of the Great Red Spot showed considerable internal structure and a "top hat" shape in which the central region rose 3 - 7 km (2 - 4 miles) above the dense, surrounding clouds. Numerous thunderheads up to 50 km (30 miles) tall were identified just north of the Spot.

The Great Red Spot as seen by Galileo. Credit: NASA/JPL

Galileo's near-infrared spectrometer was also able to detect dry, hot spots where the deeper atmosphere was visible through localised thinning of the main cloud deck.

Other targets of interest were the auroras of Io and Jupiter, the Io torus (a doughnut-shaped cloud of charged particles surrounding its orbit), the dark, dusty rings, some of the smaller moons, and the huge magnetosphere.

Towards the end of its marathon mission, Galileo took part in a unique experiment to explore the Jovian environment. Taking advantage of the fleeting flyby of the Cassini spacecraft, en route to Saturn, scientists organised a joint observational campaign to study the magnetosphere and its interaction with the solar wind. The results provided new insights into the complex bubble of charged particles that surrounds the giant planet.

Grand finale
The final phase of Galileo's epic journey began on November 5, 2002 when the orbiter flew past Amalthea and through part of Jupiter's gossamer ring before beginning its 35th and last orbit around the planet.

During this farewell loop through the Jovian system, Galileo travelled to a point more than 26 million km (16 million miles) from Jupiter - the farthest it had been in almost eight years. It then started the return leg, accelerating towards its doom.

Despite a delayed start and several, potentially devastating, handicaps along the way, Galileo has proved to be one of the most successful planetary missions ever flown, not only in terms of its longevity, but also for the myriad scientific discoveries that have shed new light on the remarkably complex Jovian system.

Swooping through Jupiter's monumental magnetosphere and belts of trapped energetic particles, the orbiter has received more than four times the cumulative dose of radiation it was designed to withstand. Despite this unprecedented battering, Galileo has operated for almost eight years in Jupiter's proximity and completed 34 encounters with Jupiter's moons.

"It has been an astonishing mission, a tribute to the ability of the ground team to overcome the obstacles and achieve a great result," said former Galileo project manager, Eilene Theilig.

Award-winning author Peter Bond is a press officer for the Royal Astronomical Society and a consultant to the European Space Agency.

Spaceflight Now staff contributed to the report.

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